This invention relates generally to apparatus and methods for measuring water content by neutron thermalization. This invention relates more specifically to a particular apparatus and method for measuring water content of a cement slurry to be used in an oil or gas well.
In the oil and gas industry, it is often necessary to know the water concentration of a fluid being pumped through a pipe. This is particularly needed for a cement slurry which is to be pumped into a well for cementing a casing or a liner into place and for a fracturing fluid which is to be pumped into a well for fracturing a formation. Presently, water content of cement slurries and fracturing fluids is determined based on measured density.
Measurements of water content based on density are, however, not necessarily accurate indications of actual water content in lightweight or air-entrained fluids. In these types of fluids, a small change in density can be related with a large change in water content. To measure the large change in water content, the small change in density would have to be measured accurately. This is particularly difficult to do with the density measuring devices presently used in the oil and gas industry.
Rather than rely on density measurements which might not have the precision or accuracy needed for accurate water content determinations, it would be desirable to measure the water content directly. In a system where water is the only source of hydrogen, water content can be determined by sensing the attenuation of neutrons emitted across a short distance of the material because the attenuation is proportional to the number of hydrogen atoms in the material. The neutron attenuation technique has, however, the following shortcomings with regard to measuring water content of a cement slurry or fracturing fluid to be used in an oil or gas well: low response (i.e., a low count rate per unit neutron emitted per second) and interference from other elements in the slurry or fluid.
Particularly for materials with relatively low water content, microwave systems have been used to measure water content directly. This technique can be used where the ionic content of the water is relatively constant and the electrical conductivity of the solids in the material is low.
In view of the foregoing, there is the need for an improved technique for measuring water content of a substance, such as a cement slurry or fracturing fluid to be used in an oil or gas well.